The invention relates to ferroelectric materials. More specifically, the invention relates to ferroelectric materials with patterned domain structures.
Nonlinear materials are used in a variety of technologies including data storage, display and communications technologies. Nonlinear materials and their effects with interacting electromagnetic radiation is well documented. Nonlinear materials are used as harmonic generators. Most commonly, nonlinear materials are used to generated the second harmonic emission light wave xcexe of an interacting light source with a fundamental wavelength xcexi. FIG. 1, for example, shows a single pass second harmonic generator construction 100. A solid state infrared laser 101 emits light with a fundamental wavelength 107. The light wave 107 is focused with a confocal lense 103 on a crystal 104 that is formed from a nonlinear material. The emission second harmonic wavelength 109 is half of the fundamental wavelength 107; equivocally the second harmonic output frequency is twice that of the fundamental input frequency. The nonlinear crystal 104 needs to be transparent to incident light with a wavelength 107 so that the light wave 107 can propagate through the crystal 104. Further, the crystal 104 needs to be transparent the to second harmonic light with a wavelength 109 so that the second harmonic light wave 109 is emitted from the crystal 104.
There are several factors that lead to inefficient conversion of the fundamental wave length 107 to the second harmonic wavelength 109. Specifically, low nonlinear coefficient of crystal material, defects in the crystal structure, low transparency of the nonlinear material, and other geometric considerations of the crystal can all lead to inefficient conversion of the fundamental wavelength 107 to the second harmonic wavelength 109. A crystal structure that is made from a material with a small nonlinear coefficient can in theory be compensated for by increasing the crystal pass length L. In practice, however, local defects and variations in refractive index throughout the crystal 104 begin to diminish any benefits gained from extending the crystal path length.
Even when the crystal 104 is formed from a material that exhibits a large nonlinear coefficient, the actual observed conversion efficiency of the fundamental wavelength 107 to its corresponding harmonic wavelength 109 is typically low. This is because light with a wavelength 107 and 109 exhibits different indices of refraction within the crystal 104. Hence, the fundamental wavelength 107 and the harmonic wavelength 109 have different phase velocities as they propagate through the crystal 104. Consequently, as the second harmonic wave 109 is locally generated in one portion of the crystal, it will be out of phase with the fundamental wavelength 107 and with the second harmonic wave 109 that is locally generated in a later part of the crystal 104 resulting in destructive interference and low output of the second harmonic light. To help overcome this problem, nonlinear materials are modified. Nonlinear materials are modified either so that the phase velocities of xcexe and xcexi are matched, a method referred to a bifringent phase matching, or alternatively the nonlinear materials are modified such that the sign of the nonlinear coefficient is periodically modulated by a distance corresponding to the coherence length of the light, a method referred to a quasi-phase matching (QPM) and described in an early work by J. A. Armstrong, N. Bloembergen, J. Ducuing and P. S. Pershan in xe2x80x9cInteraction Between Light Waves in a Nonlinear Dielectric,xe2x80x9d Phys. Rev., 127, 1918, 1962.
QPM is a method which compensates for the differences in the phase velocity between the fundamental wavelength of the interacting light source and the corresponding harmonic wavelength within the nonlinear crystal. In quasi-phase matching, the fundamental wave and the harmonic wave still have different phase velocities, but they are shifted xcfx80 out of phase relative to one another over the coherence length. The coherence length is used to refer to the distance over which two traveling waves slip out of phase by xcfx80 radians. The sign of the non-linear coefficient is reversed once every coherence length (or odd multiples of coherence lengths) causing a locally generated harmonic field within the nonlinear structure to transfer power to the harmonic beam. By compensating for the phase velocity mismatch between the fundamental wave and the harmonic wave in this way, all the elements of the crystal nonlinear tensor can be accessed throughout the entire transparency range of the crystal. This invention is directed to improved materials and methods for making quasi-phase matching structures preferably for use in non-linear optics.
The invention provides a method for domain patterning of nonlinear ferroelectric materials. The method is particularly useful for domain patterning of ferroelectric structures which exhibit low coercive fields and which exhibit charging with small changes in temperature. The method seeks to reduce the formation of random micro-domains that typically result during thermal cycling of ferroelectric materials and which lead to patterning defects and reduced efficiencies. According to the preferred method of the invention, a ferroelectric structure is provided with conductive layers on the top surface and the bottom surface of the structure which correspond to surfaces that are normal to the crystallographic polarization axis or z-polarization vectors. The conductive layer is a conductive polymer, a metal layer or a layer of conductive polymer composition. Preferably, the conductive layers are formed from a mixture of polyaniline salt, n-Methyl pyrrolidone and Isopropanol, available under the name of ORMECON(trademark) D-1000 manufactured by Ormecon Chemie GmbH and Co. KG, Ferdinand-Harten-Str. 7, D-22949, Ammersbek, Germany.
A mask is provided over a patterning surface of the structure. For simplicity, the patterning surface is referred to herein as the top surface of the structure. The mask preferably substantially replicates the intended domain pattern. Portions of the conductive layer on the top surface of the structure are removed in accordance with the pattern of the mask, thus leaving a conductive domain template on the top surface of the structure. Subsequently, a sufficient bias voltage is applied to the conductive domain template and the conductive layer on the bottom surface of the structure, thereby producing a domain patterned ferroelectric structure. The conductive layer, the mask and the conductive domain template are then preferably removed from the structure. The resulting domain patterned ferroelectric structure is then relatively stable against charging effects due to temperature variations. A final protective conductive coating may be applied to provide additional long-term stability of the domain pattern.
The mask is preferably provided by lithographic techniques by using lithographic materials. Accordingly, a portion of the conductive layer on the top surface of the ferroelectric structure is coated with a photo-resist such by any suitable method. After the photo-resist is coated on the top conductive layer, the photo-resist is thermal cycled in accordance with the manufacturer""s recommendations. The photo-resist is then exposed according to a predetermined pattern with a suitable light source and developed to form the mask.
During thermal cycling of the photo-resist, charging on the surfaces of the ferroelectric typically occurs leading to electron emission and random domain formation during cooling. In order to mitigate the charging of the structure during thermal cycling of photo-resist, it is preferable that the conductive layers on the top surface and the bottom surface are placed in electrical communication prior to-thermal cycling, thus reducing the charging. The top and bottom conductive layers are preferably placed in electrical communication by providing a conductive layer to a side surface of the ferroelectric structure.
After the mask is formed and prior to creating the domain patterning, the conductive layer on the top and bottom surfaces of the structure is placed in electrical isolation by removing the conductive layer from the side surface of the structure and applying a sufficient bias voltage across the top and bottom conductive layers. This urges the ferroelectric structure to assume a single domain structure, wherein the signs of the polarization vectors are in one direction throughout the structure. The voltage that is required to uniformly polarize the structure depends on the ferroelectric material used, but is approximately 21 KV/mm or less for many ferroelectric materials and is defined by the coercive field Ec of the material used to form the structure and the thickness of the structure.
After the mask is formed and the structure is uniformly polarized, portions of the conductive layer on the top surface are removed in accordance with the mask to form a conductive domain template. A sufficient reverse bias voltage is then applied across the conductive domain template and the conductive layer on the bottom surface of the ferroelectric structure causing the regions of the structure between the domain template and the conductive layer on the bottom surface to reverse their polarization, thereby creating the domain patterning throughout the ferroelectric structure.
The ferroelectric structure is preferably formed from LiNbO3, KTiOPO4 and LiTaO3. Most preferably, the ferroelectric structure is a stoichiometric LiNbO3 or LiTaO3 wafer which exhibits a low coercive field. Further, the domain patterned ferroelectric structure is preferably a quasi-phase matching structure wherein the domains are spatially modulated by a distance corresponding to a coherence length required for generating a harmonic emission wave form with a wavelength xcexe from a fundamental wave form of an interacting light source with a wavelength xcexi.
A harmonic generator for generating a harmonic emission wave form utilizes the quasi-phase matching structure of the instant invention formed from a ferroelectric material which exhibits spontaneous reversal of local polarizations by changes in temperature xcex94T between 0.1 and 40 degrees, wherein xcex94T,=qxe2x88x921xc2x7"xgr"xc2x7Ec, q is the pyroelectric coefficient, "xgr" is the permitivity of the ferroelectric and Ec is the coercive field. An interacting light source, with the fundamental wavelength xcexi is configured to be incident with the quasi-phase matching structure such that a portion of the light with the wavelength xcexi interacts with the quasi-phase matching structure generating the harmonic emission wave form with a wavelength xcexe.